Question

Select the incorrect statement from the following.

• A. \(\text{NH}_3\) and \(\text{NF}_3\) both are polar molecules
• B. \(\text{NH}_3\) and \(\text{NF}_3\) both have pyramidal shape
• C. In \(\text{NH}_3\) and \(\text{NF}_3\) the orbital dipole due to lone pair is in the same direction as that of resultant dipole moment of other three bonds
• D. Dipole moment of \(\text{NH}_3\) is greater than \(\text{NF}_3\)

Hint:

In NH3 the lone-pair moment adds to the bond moments, but in NF3 it opposes them.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Both \(\text{NH}_3\) and \(\text{NF}_3\) have nitrogen with three bond pairs and one lone pair. The shape is pyramidal and both molecules are polar. What differs is the direction of the bond dipoles compared with the lone-pair dipole.

Step 2: Key Formula or Approach:
Net dipole moment is the vector sum of the bond dipoles and the lone-pair (orbital) dipole. The lone pair on N points away from the nitrogen, along the axis of the pyramid.

Step 3: Check statement (A):
Both molecules are pyramidal with unequal charge distribution, so both are polar. (A) is TRUE.

Step 4: Check statement (B):
Both have \(sp^3\) nitrogen with one lone pair, giving a trigonal pyramidal shape. (B) is TRUE.

Step 5: Check statement (C):
In \(\text{NH}_3\), N is more electronegative than H, so the N-H bond dipoles point towards N. Their resultant points the same way as the lone-pair dipole, so the two add up.
In \(\text{NF}_3\), F is more electronegative than N, so the N-F dipoles point towards F. Their resultant is opposite to the lone-pair dipole and the two partly cancel.
So the claim that the lone-pair dipole is in the same direction as the bond-dipole resultant in both molecules is FALSE.

Step 6: Check statement (D):
Because of the addition in \(\text{NH}_3\) (about 1.46 D) and the opposition in \(\text{NF}_3\) (about 0.24 D), the dipole moment of \(\text{NH}_3\) is larger. (D) is TRUE.

Final Answer:
Statement (C) is the incorrect one, because in \(\text{NF}_3\) the lone-pair dipole opposes the resultant of the bond dipoles. \[ \boxed{\text{(C)}} \]